It has been common practice in the past in the preparation of dinitro aromatic compounds to use concentrated nitric acid containing sulfuric acid as a catalyst, as the nitrating medium. This is as exemplified, in U.S. Pat. No. 4,136,117 to Robert E. Diehl and Stephen D. Levy, issued Jan. 23, 1979. Also it is known from the same patent (col. 1, lines 52-60) that Belgium Pat. No. 762,232 discloses a method for the preparation of 2, 6-dinitro-tertiary anilines, wherein both N-substituents are haloalkyl, by nitration with at least a five fold excess of nitric acid, which is present at the start of the reaction in a concentration of 50% to 90% and in an amount to leave an acid concentration of 50% at the end of the reaction, in the presence of a catalytic amount of nitrous acid or nitrite ion generating material.
Each of the above processes have disadvantages from an economical and environmental standpoint. The processes utilizing mixtures of nitric acid and sulfuric acid generate spent acids containing 40% or more acid, including some nitric acid. This spent acid either has to be neutralized prior to disposal--resulting in the generation of considerable amounts of mixtures of sodium nitrate and sodium sulfate which must be disposed of in the environment--or the spent acid must be concentrated by evaporation of water (with expenditure of energy) to a concentration which can be used for subsequent nitration reactions.
In the other process, referred to above, in which a five fold excess of concentrated nitric acid is utilized along with a nitrous acid or nitrite ion generating material, the spent acid from the dinitration process contains 50% or more of nitric acid. This spent acid, like that containing sulfuric acid, must either be concentrated prior to use in the dinitration process or must be neutralized, for example, to form sodium nitrate, and then disposed of in the environment.